Question

What were some of the reasons that the united states became involved in the vietnam war?

Answer 1

One could be the idea of the "Domino Theory" in the Far East.

Explanation:

The US administration started to become nervous about Soviet expansion in particular in the Far East during the Korean War.
It was clear the USSR was not going to remain idle letting the American control the back garden of Russia, i.e., Asia.

At this point was forged the idea of the Domino Theory; small nations in Asia, such as Vietnam or Cambodia, could decide to get a communist government supported by the Soviet Union becoming, basically, an ally of the Russians; if left unchecked, this could become an accelerating tendency involving neighboring countries as in a domino effect where if you tilt one domino tile, while falling, it will touch the next one making it fall as well and repeating the process for all the others.

In practice if Vietnam was left alone and free to get rid of the French and acquire a communist government soon other countries like Cambodia, Laos, Thailand, etc. were inevitably going to follow suit to become communist countries.